As cloud services and big data utilization are popularized, information is transmitted through the network in larger and larger quantities. In response to such an increase in quantity of information transmitted, there has been widely spread so-called optical communication, which is information transmission by use of optical signals. Information transmission by use of optical signals allows a further increase in capacity of a transmission path and in transmission speed than information transmission by use of electrical signals.
Optical communication is carried out by use of optical signals obtained by modulating light with electrical signals indicative of information. The optical signals are generated by use of an optical modulator in optical communication. Of various types of existing optical modulators, a Mach-Zehnder (M-Z) optical modulator is often used as an optical modulator for use in optical communication. This is because the M-Z optical modulator has advantages of (i) being less likely to be affected by external noise and (ii) being highly stable in modulation operation with respect to a change in temperature.
The M-Z optical modulator generally includes: an input-side waveguide; an M-Z optical interferometer including an optical modulating section; and an output-side waveguide configured to output modulated light. The M-Z optical interferometer includes two arm sections branching from the input-side waveguide. At least one of the arm sections is provided with an electrode for modulating light. To this electrode, a signal line is connected. The signal line has (i) a first end that is connected to a pulse pattern generator configured to generate a modulating signal and (ii) a second end that is connected to a termination resistor. The termination resistor is provided to suppress the reflection, at the second end of the signal line, of a modulating signal inputted from the first end of the signal line.
The termination resistor is normally equipped by either a method for mounting a resistor on a surface of a substrate of the optical modulator or a method for mounting a resistor on a surface of another substrate electrically connected with the second end of the signal line of the optical modulator. Alternatively, the termination resistor can also be equipped by a known method for forming a resistive element inside a substrate of the optical modulator and connecting the resistive element to the second end of the signal line. Non-patent Literature 1 discloses, for example, a technique in which titanium nitride (TiN) formed inside a silicon (Si) substrate is used as a resistive element. Further, Patent Literatures 1 and 2 each disclose a technique for forming a resistive element by doping an inside of a silicon layer of an SOI (silicon on insulator) wafer with an element (not Si). The method for forming a resistive element inside a substrate is more advantageous than the method for mounting a resistor on a surface of a substrate. This is because the method for forming a resistive element inside a substrate allows the resistive element to use a smaller space on the surface of the substrate and makes it unnecessary to equip the surface of the substrate with a resistive element as a component.